This proposal is in response to the cancer prevention research small grant program. We will examine the feasibility of p27/Kip1 deficiency in transgenic mice for potential use in mammary carcinogenesis studies in chemopreventive studies. The development of cancer is a multi-step process involving alterations in genes and gene families. Among them, the genes (p53, p21, and p27) appear to have principal roles. In this project, we will examine the effect of p27 deficiency in a transgenic mouse model on mammary cell proliferation and carcinogenesis. P27/Kip1 is a cell cycle suppressor that inhibits cyclin D1-D3/cdk4 and or cyclin E/cdk2 activity in the G1 phase of the cell cycle and thus prevents the G1-S transition and cell-type progression. Several clinical studies have shown that p27-/Kip1 is not only a cell cycle suppressor but also a tumor suppressor. p27/Kip1 is down-regulated in most malignant tumors, including breast cancer, and the level of p27 expression might be used as an independent prognostic marker of tumor progression. The molecular mechanisms underlying the suppressor role of p27/Kip1 in breast cancer development are poorly understood. Here, we will assess the role of p27/kip1 deficiency in transgenic mice on estradiol and progesterone (E+P)-induced cell proliferation in mammary epithelial cells, as well as on mammary cell transformation and carcinogenesis. Dimethylbenz-(a)-anthracene (DMBA) will be used to induce transformation on mammary epithelial cells (MECs) in vitro and to initiate mammary carcinogenesis in p237 transgenic (p27 +/+, +/-, -/-) mice. We will test the hypothesis that a loss of decrease of p27/kip1 expression will promote the E+P-induced proliferation of mammary epithelial cells as well as DMBA-induced mammary cell transformation and carcinogenesis. The specific aims designed to test our hypothesis are as follows. 1. Examine the role of p27/kip1 deficiency on E+P -induced cell proliferation in mammary gland. 2. Examine the role of p27 deficiency can modulate the in vitro transformation of mammary epithelial cells. A mouse mammary organ culture (MMOC) system will be used for in vitro studies. 3. Assess whether p27 deficiency can promote DMBA-induced mammary carcinogenesis administration and during the promotion phase, the animals will be treated with E+P starting one week before carcinogen administration and continued for 8 weeks after administration. The results from this study will show the feasibility of the p27-deficient of the p27-deficient mouse model for future studies on mammary carcinogenesis as well as for assessing the efficiency of chemoprevention agents in short-term in vitro studies and long-term animal experiments.